1. Field of the Invention.
The invention relates to the transmission of data in digital form over lines.
2. Prior Art.
Frequently, digital data is transmitted over lines at a fixed rate. Numerous standard formats are used for this transmission such as the so-called "T1" format where data is transmitted at a rate of 144K bits per second.
The receiver must synchronize itself with the rate at which the data is received in order to properly sense the data. Often, the line and related input parameters are first examined to determine electrical characteristics. This allows the receiver to adjust its characteristics such as the frequency at which it receives data and the phase of this frequency, and to compensate for the delay inherent in the line and the frequency versus attenuation characteristics of the line. It also allows the adjustment of hybrid circuits for balance.
Typically, however, the characteristics of the line vary, albeit slowly. This slow variation is most often found in lines without repeaters, although it can occur in lines with stable repeaters. This variation can be due to atmospheric conditions such as temperature, wind, or moisture.
There are several methods and apparatuses for reexamining the characteristics of the line and related input parameters while the data is being transmitted. These are generally quite complex and involve mathematically derived solutions which are based on certain assumptions such as the randomness of the data. In other cases, adjustments are made to achieve the lowest error rate which is also complex.
As will be seen, the present invention provides a relatively simple method and apparatus for permitting the transmission of the data and the examination of the characteristics of the line and related input characteristics.